1. Field of the Invention
The invention relates generally to the field of drilling wellbores below the bottom of a body of water such as a lake or an ocean. More particularly, the invention relates to methods for stopping uncontrolled flow of fluids from such wells in the event existing fluid flow control devices fail.
2. Background Art
Drilling wellbores into rock formations below the bottom of a body of water from a lake or ocean includes disposing a mobile offshore drilling unit (MODU) above the water surface, typically above the place on the water bottom where the wellbore drilling is started. The MODU deploys equipment to drill a “surface hole”, or a portion of the wellbore from the water bottom to a selected depth below the water bottom. Once the depth of the surface hole is reached, a pipe called a “surface casing” is typically inserted and cemented in place. For further drilling of the wellbore to selected formations, e.g., in which hydrocarbons are believed to be present, a device called a “blowout preventer stack” (hereinafter BOP) is typically affixed to a flange or similar connector disposed at the top of the surface casing. See, e.g., U.S. Pat. No. 6,554,247 issued to Berckenhoff et al. for description of an example of a BOP.
The BOP includes one or more “rams” or devices which may be close to form a pressure tight seal, typically by application of hydraulic pressure to actuators for the rams. The rams are provided to hydraulically close the well in the event the well is drilled through formations having fluid pressure therein which exceeds the hydrostatic or hydrodynamic pressure of fluid (“drilling mud”) used to drill the wellbore. In such occurrences, it is known in the art that entry of formation fluids into the drilling mud, particularly natural gas, can alter the drilling mud pressure in the wellbore, thus allowing additional fluid to enter the wellbore. The BOP may be operated in such circumstances to prevent uncontrolled discharge of fluid from the formation into the wellbore, while the fluid pressure in the wellbore is adjusted from the MODU. See, e.g., U.S. Pat. No. 6,499,540 issued to Schubert et al. and U.S. Pat. No. 6,474,422 issued to Schubert et al. for an explanation of circumstances leading to the need to operate the BOP and how to safely remove the fluid that has entered the wellbore.
The MODU may be a floating drilling platform (e.g., a semisubmersible platform or drillship) that is not supported from a structure extending to the water bottom. Drilling from a floating drilling platform typically includes installing a pipe from the MODU at the water surface to a connection therefore on the BOP called a “riser.” It is also known in the art to drill wellbores below the water bottom without a riser. See, e.g., U.S. Pat. No. 4,149,603 issued to Arnold. It is also known in the art to use water bottom supported MODUs (e.g., “jackup” drilling units) for drilling wellbores below the water bottom.
Irrespective of the type of MODU used or whether the drilling system includes a drilling riser, subsea drilling including the use of a BOP system proximate the water bottom mounted on the surface casing typically includes a plurality of hydraulic pressure accumulators charged to a selected pressure, control valves and other devices so that the BOP system may be operated from controls disposed on the MODU. The controls send electrical and/or hydraulic control signals to the control valves to actuate the various elements of the BOP when needed. See the Berckenhoff '247 patent, for example.
Most government agencies having regulatory authority over drilling operations of the type described above require that the BOP system is tested at certain times to ensure correct operation. Despite these requirements, and despite best efforts of MODU contractor entities to ensure correct operation of BOPs, BOPs have been known to fail. Such failure may be accompanied by catastrophic destruction of property, including total loss of the MODU, injury to persons and loss of life. Further, in such circumstances, including if the MODU is lost, uncontrolled discharge of fluids from the subsurface formations may take place for an extended period of time while equipment to close in or “cap” the well is located and deployed on the wellbore location. Such uncontrolled discharge may lead to substantial environmental damage. Further, methods known in the art for capping a wellbore with a failed BOP require securing another MODU and moving it to the location, with accompanying risk of property damage and risk to human life. Still further, such known methods rely on the use of fluid pumps on remotely operated vehicles (ROVs) to operate hydraulically operated actuators for closing the wellbore to further fluid flow. Because the pumps on a typical ROV have limited flow capacity, it may take an extended amount of time to close the hydraulically operated actuators. Taking such extended time while fluid is discharging from the wellbore risks erosion of the sealing devices, thus making known methods of capping a subsea wellbore subject to inherent failure risk.
What is needed is a method for capping a subsea wellbore having a failed BOP stack that can be operated quickly to reduce risk of seal element failure, and can be deployed from any vessel, thus eliminating the requirement to obtain another MODU in the event of loss of the MODU that drilled the well, or using another MODU to supplement the operation of any MODU still near the wellbore location.